Chap. XII] 



SUMMARY 



229 



Position and Direc- 

 tion of Valves 



Large Number and 

 Small Bore of the 

 Capillaries 



Large Size and Non- 

 contractile Charac- 

 ter of the Walls of 

 the Veins 



Distribution of Blood • 



Permit blood to flow from auricles into ventricles, 

 7iot in reverse direction. 



Permit blood to flow from ventricles into arteries, 

 not in reverse direction. 



Disease and advancing years may impair adjust- 

 ment. 



Total number is immense. 



Each one is microscopic. 



Total capacity several hundred times greater than 



aorta. 

 Rapidity of blood stream reduced by enormous 



increase in number of vessels, and in surface. 



f Begin small — grow larger. 

 Decrease in number of vessels and in total capacity. 

 Vein twice as large as corresponding artery. 

 Not capable of distention and recoil, hence offer 

 no resistance to current of blood. 



' Estimated amount of blood in body, 4 qts. 

 Estimated amount of blood in thorax, 1 qt. 

 Estimated amount of blood in skeletal muscles, 



1 qt. 



Estimated amount of blood in liver, 1 qt. 

 Amount in any organ increased when there is need. 

 Amount in any organ decreased when need is over. 



Blood 

 Pressure 



' Pressure blood exerts against walls of vessels. 



High — Hemorrhage from cut artery — 

 " spurts " out. 



■ 1. Contraction of ventricles. 



2. Extra blood forced into full 

 arteries. 



3. Semilunar valves prevent 

 regiu-gitation. 



Maintained by <! 4. Elasticity and recoil of 

 arteries. 



5. Resistance offered by arte- 

 rioles. 



6. Increased surface in capil- 

 laries. 



f 1. Higher during systole. 

 I 2. Lower during diastole. 

 Not uniform { 3. Increases with age. 



I 4. Decreases if heart or ar- 

 [ teries lose their tone. 



Arterial 



